Nagra Master Equalization
by Dan Dugan
Most Nagra users ignore the second 15 ips position on the IV-S speed switch, labeled "Nagra Master." This is too bad, because it's a very useful tool. Nagra Master is a special equalization which gives a greatly improved signal-to-noise ratio at 15 ips compared to the NAB standard, and a "velvety" rather than "hissy" noise background. With current high-level tapes like Quantegy 480 it compares favorably with digital recording.
Recording at the standard dialogue speed of 7.5 ips requires a substantial treble boost (preemphasis) inside the recording amplifier. This is to compensate for losses in the recording process. This treble boost reduces the high-frequency headroom. It is possible to record all the way up to +4 modulation at lower frequencies, but by the time you're up to 10 KHz, the boost has hit the ceiling, so though the frequency response is flat at low and moderate signal levels, high treble levels will saturate. The maximum recorded level at 10 KHz with today's tapes is five to ten dB lower than the low-frequency maximum. This is not a problem with voices or classical music, because the peak energy spectrum of these sources also rolls off at higher frequencies. Also, at 7.5 ips there is no complementary rolloff in the playback, as there is in the RIAA disk equalization, for example, so an overall phase shift with frequency , technically called group delay, is an inherent sonic limitation of 7.5 ips recording.
At 15 ips, no treble boost is needed. Indeed, with today's tapes, a small amount of treble rolloff is required. This makes the 15 ips NAB record EQ good for recording hot cymbals or synthesizers, or treble-rich sound effects like hissing steam, that would have to be turned down to avoid treble saturation at 7.5 ips. Not having the preemphasis at 15 ips also means much less group delay.
Kudelski's trick with Nagra Master is to use the 7.5 ips treble boost for recording at 15 ips. Then a complementary treble rolloff is used in playback to restore flat frequency response. This makes a dramatic decrease in the tape noise level, and furthermore, shapes the noise spectrum to a velvety hush rather than an intrusive hiss. This has to be heard to be appreciated. Since the preemphasis and playback rolloff are complementary, the group delay (phase shift with frequency) stays low. Nagra Master is more like digital than any other analog system.
There are two disadvantages to using Nagra Master. First, you have to be aware of the possibility of high-frequency saturation due to the preemphasis. This is no problem if you're used to recording at 7.5 ips. The preemphasis is the same, so the same recording level habits will serve. The second problem is confusion at the transfer house. You have to make sure everybody down the line knows how to handle Nagra Master before you use it for a project. The tapes must be transferred either from a Nagra (IV-S or T-Audio), or from a standard NAB machine followed by a Nagra Master rolloff EQ.
Recordists "in the know" about Nagra Master have achieved spectacular results. Nelson Stoll used it for "The Black Stallion" and "Dune." Steve Powell used Nagra Master EQ for the series "Midnight Caller."